Cuando corro el apt-mirror, antes de comenzar la descarga, me pone esto:
Processing indexes: [PPsh: 1: cannot open ubuntu.uci.cu/ubuntu//dists/xenial-security/universe/binary-amd64/Packages.gz: No such file
apt-mirror: can’t open index in process_index_gz at /usr/bin/apt-mirror line 769.
PPsh: 1: cannot open ubuntu.uci.cu/ubuntu//dists/xenial-proposed/restricted/binary-amd64/Packages.gz: No such file
apt-mirror: can’t open index in process_index_gz at /usr/bin/apt-mirror line 769.
PPPsh: 1: cannot open ubuntu.uci.cu/ubuntu//dists/xenial-security/multiverse/binary-i386/Packages.gz: No such file
apt-mirror: can’t open index in process_index_gz at /usr/bin/apt-mirror line 769.
PPsh: 1: cannot open ubuntu.uci.cu/ubuntu//dists/xenial-proposed/universe/binary-i386/Packages.gz: No such file
apt-mirror: can’t open index in process_index_gz at /usr/bin/apt-mirror line 769.
P]

No se si esto repercute en algo o no. Pense en descargar esos archivos manualmente.